WASHINGTON — The Bush administration announced yesterday it will require states to report high school graduation rates in a uniform way instead of using a variety of methods that critics say are often based on unreliable information.

Education Secretary Margaret Spellings announced the change at a news conference at which a report was released showing that 17 of the nation’s 50 largest cities had high school graduation rates lower than 50%. The change involves the No Child Left Behind Act, which currently allows states to use their own methods of calculating graduation rates and set their own goals for improving them.

The report by the America’s Promise Alliance, using a common method to evaluate graduation rates for cities, found the lowest graduation rates in Detroit, Indianapolis, and Cleveland.

It found that about half of the students served by public school systems in the nation’s largest cities receive diplomas; students in suburban and rural public high schools were more likely to graduate than their counterparts in urban public high schools.

Nationally, about 70% of American students graduate on time with a regular diploma and about 1.2 million students drop out annually.